1. Field of the Invention
The present invention relates to a clamping circuit for our use in a color television circuit, and more particularly, to a high impedance clamping circuit for use in digital television circuit.
2. Related Art
FIG. 1 shows a standard NTSC composite video signal. According to IRE (Institute of Radio Engineers) standard, 1 Volt Peak to Peak Video is divided up into 140 IRE units. This is done to make numbers for luminance levels easier to communicate. The amplitude of the video signal from blanking (zero volts) to peak white is 0.714286 volts or 100 IRE units. Synchronization signals extend from blanking to −1.285714 volts/−40 IRE units.
A horizontal sync is the −40 IRE pulse occurring at the beginning of each line. This pulse signals the electronic beam of the picture monitor to go back to the left side of the screen and trace another horizontal line picture information. The horizontal sync (HSYNC) signals the beginning of each new video line.
Luminance is a signal which represents brightness, or the amount of light in the picture. This is the only signal required for black and white pictures, and for color systems it is obtained as a weighted sum (Y=0.3R+0.59G+0.11B) of the R, G and B signals.
Chroma is the characteristics of color information, independent of luminance intensity. Hue and saturation are qualities of chroma. The chroma signal consists of two quadrature components modulated on to a carrier at the color burst frequency. The phase and amplitude of these components determine the color content at each pixel.
The horizontal sync is followed by a back porch, which is used as a reference level to restore the DC component (i.e., brightness) of an otherwise floating AC-coupled video signal. This process of DC level restoration is named “clamping”, and takes place during the back porch. In color systems, a burst (called “color burst”) of sub-carrier frequency is located on a back porch of the composite video signal, as shown in FIG. 1. The back porch represents a black level of the video signal, while the color burst serves as a color synchronizing signal to establish a frequency and phase reference for the chrominance signal. Thus, the color burst is a high-frequency region that provides a phase and amplitude reference for the subsequent color information.
A clamp is a circuit that restores the DC component of a signal. A video clamp circuit, usually triggered by the horizontal synchronizing pulses, re-establishes a fixed DC reference level for the video signal. A major benefit of clamping is the removal of low-frequency interference, especially power line hum.
In a digital system, an A/D converter must sample the analog composite signal before the luma and chroma are separated into their respective channels. The clamp, or a DC restore circuit, is needed to keep the analog signal within the dynamic range of the A/D converter. The blank level is measured during the color burst area (after a digital low pass filter), but the clamp pulse cannot be applied to the incoming analog signal during this same time. The analog burst must be maintained because it is used to demodulate the chroma into its baseband signal. Adding a pulse during the back porch is difficult due to tolerance on the 10.8 μs from sync to active video and extra long bursts for copy protection (Macrovision).